11 Visual effects


11.1 Overflow and clipping

Generally, the content of a block box is confined to the content edges of the box. In certain cases, a box may overflow, meaning its content lies partly or entirely outside of the box, e.g.:

Whenever overflow occurs, the 'overflow' property specifies whether a box is clipped to its content box, and if so, whether a scrolling mechanism is provided to access any clipped out content.

11.1.1 Overflow: the 'overflow' property

Value:  visible | hidden | scroll | auto | inherit
Initial:  visible
Applies to:  block-level and replaced elements
Inherited:  no
Percentages:  N/A
Media:  visual
Computed value:  as specified

This property specifies whether the content of a block-level element is clipped when it overflows the element's box (which is acting as a containing block for the content). Values have the following meanings:

This value indicates that content is not clipped, i.e., it may be rendered outside the block box.
This value indicates that the content is clipped and that no scrolling mechanism should be provided to view the content outside the clipping region; users will not have access to clipped content.
This value indicates that the content is clipped and that if the user agent uses a scrolling mechanism that is visible on the screen (such as a scroll bar or a panner), that mechanism should be displayed for a box whether or not any of its content is clipped. This avoids any problem with scrollbars appearing and disappearing in a dynamic environment. When this value is specified and the target medium is 'print', overflowing content should may be printed.
The behavior of the 'auto' value is user agent-dependent, but should cause a scrolling mechanism to be provided for overflowing boxes.

Even if 'overflow' is set to 'visible', content may be clipped to a UA's document window by the native operating environment.

HTML UAs may apply the overflow property from the BODY or HTML elements to the viewport.


Consider the following example of a block quotation (BLOCKQUOTE) that is too big for its containing block (established by a DIV). Here is the source document:

<P>I didn't like the play, but then I saw
it under adverse conditions - the curtain was up.
<CITE>- Groucho Marx</CITE>

Here is the style sheet controlling the sizes and style of the generated boxes:

div { width : 100px; height: 100px;
      border: thin solid red;

blockquote   { width : 125px; height : 100px;
      margin-top: 50px; margin-left: 50px; 
      border: thin dashed black

cite { display: block;
       text-align : right; 
       border: none

The initial value of 'overflow' is 'visible', so the BLOCKQUOTE would be formatted without clipping, something like this:

Rendered overflow

Setting 'overflow' to 'hidden' for the DIV element, on the other hand, causes the BLOCKQUOTE to be clipped by the containing block:

Clipped overflow

A value of 'scroll' would tell UAs that support a visible scrolling mechanism to display one so that users could access the clipped content.

11.1.2 Clipping: the 'clip' property

A clipping region defines what portion of an element's border box is visible. By default, the clipping region has the same size and shape as the element's border box. However, the clipping region may be modified by the 'clip' property.

Value:  <shape> | auto | inherit
Initial:  auto
Applies to:  absolutely positioned elements
Inherited:  no
Percentages:  N/A
Media:  visual
Computed value:  For rectangle values, a rectangle consisting of four computed lengths; otherwise, as specified

The 'clip' property applies only to absolutely positioned elements. Values have the following meanings:

The clipping region has the same size and location as the element's box(es).
In CSS 2.1, the only valid <shape> value is: rect (<top>, <right>, <bottom>, <left>) where <top> and <bottom> specify offsets from the top border edge of the box, and <right>, and <left> specify offsets from the left border edge of the box in left-to-right text and from the right border edge of the box in right-to-left text. Authors should separate offset values with commas. User agents must support separation with commas, but may also support separation without commas, because a previous version of this specification was ambiguous in this respect.

<top>, <right>, <bottom>, and <left> may either have a <length> value or 'auto'. Negative lengths are permitted. The value 'auto' means that a given edge of the clipping region will be the same as the edge of the element's generated box (i.e., 'auto' means the same as '0' for <top> and <left> (in left-to-right text, <right> in right-to-left text), the same as the computed value of the height plus the sum of vertical padding and border widths for <bottom>, and the same as the computed value of the width plus the sum of the horizontal padding and border widths for <right> (in left-to-right text, <left> in right-to-left text), such that four 'auto' values result in the clipping region being fit with the border edge).

When coordinates are rounded to pixel coordinates, care should be taken that no pixels remain visible when <left> and <right> have the same value (or <top> and <bottom> have the same value), and conversely that no pixels within the element's box remain hidden when these values are 'auto'.

The element's ancestors may also have clipping regions (e.g. if their 'overflow' property is not 'visible'); what is rendered is the intersection of the various clipping regions.

If the clipping region exceeds the bounds of the UA's document window, content may be clipped to that window by the native operating environment.


The following two rules:

p { clip: rect(5px, 40px, 45px, 5px); }
p { clip: rect(5px, 55px, 45px, 5px); }

will create the rectangular clipping regions delimited by the dashed lines in the following illustrations:

Two clipping regions

Note. In CSS 2.1, all clipping regions are rectangular. We anticipate future extensions to permit non-rectangular clipping. Future versions may also reintroduce a syntax for offsetting shapes from each edge instead of offsetting from a point.

11.2 Visibility: the 'visibility' property

Value:  visible | hidden | collapse | inherit
Initial:  visble
Applies to:  all elements
Inherited:  yes
Percentages:  N/A
Media:  visual
Computed value:  as specified

The 'visibility' property specifies whether the boxes generated by an element are rendered. Invisible boxes still affect layout (set the 'display' property to 'none' to suppress box generation altogether). Values have the following meanings:

The generated box is visible.
The generated box is invisible (fully transparent), but still affects layout. Furthermore, descendents of the element will be visible if they have 'visibility: visible'.
Please consult the section on dynamic row and column effects in tables. If used on elements other than rows or columns, 'collapse' has the same meaning as 'hidden'.

This property may be used in conjunction with scripts to create dynamic effects.

In the following example, pressing either form button invokes a user-defined script function that causes the corresponding box to become visible and the other to be hidden. Since these boxes have the same size and position, the effect is that one replaces the other. (The script code is in a hypothetical script language. It may or may not have any effect in a CSS-capable UA.)

<STYLE type="text/css">
   #container1 { position: absolute; 
                 top: 2in; left: 2in; width: 2in }
   #container2 { position: absolute; 
                 top: 2in; left: 2in; width: 2in;
                 visibility: hidden; }
<P>Choose a suspect:</P>
<DIV id="container1">
   <IMG alt="Al Capone" 
        width="100" height="100" 
   <P>Name: Al Capone</P>
   <P>Residence: Chicago</P>

<DIV id="container2">
   <IMG alt="Lucky Luciano" 
        width="100" height="100" 
   <P>Name: Lucky Luciano</P>
   <P>Residence: New York</P>

<FORM method="post" 
   <INPUT name="Capone" type="button" 
   <INPUT name="Luciano" type="button"